Microprocessors, in the past, have been widely used to control and monitor devices operating at voltage and power levels which exceed the capabilities of the microprocessor itself. Typical examples include driving fractional horsepower DC motors and solenoids in consumer electronics, small industrial machines, automotive applications, robotic mechanical controls, etc. The use of the microprocessor as a controlling and monitoring device is extremely useful due to the flexibility and processing power of the microprocessor, and since often times the controlling input signals to the microprocessor are in digital form which are readily usable by the microprocessor. Further, the unique control program required to instruct the microprocessor may be stored in mask programmable Read Only Memory (ROM) or Electrically Programmable Read Only Memory (EPROM). Typically the outputs of the microprocessor, however, are limited to a voltage range equal to that of the microprocessor power supply, and to current drive capabilities in the range of 12 to 48 milli-amps per output driver.
Further increases in output drive capability on the microprocessor is possible by using bipolar transistors in the output circuits. The majority of microprocessors currently being used are Complementary Metal Oxide Semiconductor (CMOS) due to the high transistor densities and lower power consumption requirements. The bipolar devices, however, require a relatively large base drive current and additionally, several more masking steps in the manufacturing process. Also, bipolar devices require a large amount of silicon real estate and dissipate large amounts of power relative to several special purpose MOS power devices such as Lateral DMOS (LDMOS), Vertical DMOS (VDMOS), and Updrain DMOS (UDMOS). The high base drive current problem of the bipolar devices may be reduced somewhat by using Darlington transistor pairs in their place, but the requirements of large silicon area still exists and additionally undesirable forward voltage drops develop across the Darlington transistor pairs which make these devices unsuitable for many applications. If the bipolar output driver were driving 100 milli-amps and the transistor fell into the linear region of operation from the saturation mode, the bipolar device would rapidly heat and likely destroy the microprocessor circuits. Integrated bipolar output transistors are therefor limited in their application due to cost and power dissipation considerations.
It would be advantageous to provide a special purpose microprocessor capable of providing high current output capabilities on chip. This could provide cost savings as well as decrease the package size relative to a microprocessor driving a separate power device. This provides the ability to place a single controlling-driver chip in areas where it is not currently feasible to place a multiplicity of chips due to interconnect and mounting problems. The microprocessor also has the ability to closely monitor the temperature of the power devices and take corrective action to avoid damage caused by overheating. Adding power drivers to the microprocessor requires solving problems of high temperatures, high power dissipation, collecting excessive substrate currents, and providing higher voltages than provided by the power supply. Additionally it is desirable to have a power device that is process compatible with an existing microprocessor technology in order to utilize an existing microprocessor design.
Thus, what is needed is a microprocessor having large current drive capability, wherein integrated power devices exhibit a low drain-source on resistance (rdson) and are process compatible with the microprocessor technology.